In epidemiology, a 'propagated outbreak' occurs when:

In epidemiology, a 'propagated outbreak' refers to a scenario where the cases of disease arise from person-to-person transmission rather than from a single common source. This type of outbreak occurs gradually as individuals infect one another, leading to a sustained increase in cases over time.

The process begins with an initial case that may have been exposed to an infectious agent, which then spreads the disease to others. These subsequently infected individuals can, in turn, spread the disease to additional people, creating a chain of transmission. This characteristic defines propagated outbreaks and distinguishes them from common-source outbreaks, where individuals are exposed to the same source simultaneously, leading to a sudden spike in cases.

The other options reflect different characteristics of disease outbreaks but do not accurately define a propagated outbreak. Therefore, the correct understanding is that B captures the gradual person-to-person transmission that typifies a propagated outbreak.